EP261025a
GCN Circular 46055
Stella Manyan (Columbia U), K. De (Columbia U), G. Mo (Caltech/Carnegie), V. Karambelkar (Columbia U), D. Schiminovich (Columbia U) report:
We observed the field of EP261025a (Yang et al., GCN 46025, Mandarakas et al., GCN 46026; Mohan et al., GCN 46028) in the near-infrared J band with the MDM InfraRed Astronomy inGaas Experiment (MIRAGE) instrument on the MDM 1.3 m telescope.
Observations began at UT 2026-10-26T02:49:34 (28.2 hours after EP-WXT trigger) in the J band for a total exposure time of 3210s.
We do not detect any new source within the FXT localization error circle (Yang et al., GCN 46025). We obtain the following 5-sigma upper limit in AB magnitudes: J > 19.8 AB mag.
MIRAGE is a new robotic YJHs-band near infrared imager for the MDM 1.3 m telescope. We thank the Mt Cuba astronomical foundation for supporting development of MIRAGE and MDM Observatory staff for supporting the observations.
GCN Circular 46028
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of EP261025a (Yang et al., GCN 46025) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began on 2026-10-25 at 12:46:53.567 UTC in the J band (5.45 hr after the WXT light curve began), consisting of 21*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We do not detect any new source in the FXT localization in our stacked image, and obtain the following 5-sigma upper limit: J = 18.9 mag (AB). Optical upper limit is also reported by Mandarakas et al., GCN 46026.
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 46026
Nikos Mandarakas (LAM), Ramandeep Gill (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) report:
We imaged the field of the EP 261025a (Yang et al., GCN Circ. 46025) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-10-25T12:29:17 to 2026-10-25T12:42:25 UTC (from 5.155 to 5.374 hours after the trigger) and obtained 11 minutes of exposure in the r filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the EP-WXT source position (Yang et al., GCN Circ. 46025) down to the following 3-sigma limit:
r > 21.56
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 46025
G. J. Yang (NAO, CAS), S. Q. Zheng, J. R. Liu (BNU), Q. Y. Wu and Y. Liu (NAO, CAS), on behalf of the Einstein Probe (EP) team:
We report the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP261025a. The EP-WXT observation (ID: 08500000682) started at 2026-10-25T05:38:04 UTC, with an exposure time of 5605 s. The WXT light curve of the transient began at 2026-10-25T07:19:57.4 UTC and lasted for ~130 s. The WXT position of the source is R.A. = 2.554 deg, DEC = 60.763 deg (J2000) (l=117.917 deg, b=-1.707 deg) with an uncertainty of 2.22 arcmin in radius (90% C.L. statistical and systematic).
We fitted the WXT spectrum with an absorbed power-law model, fixing nH at the Galactic value 7.06e21 cm-2. We obtained a photon index of 1.6(-0.3/+0.3) and an unabsorbed flux of 2.6(-0.3/+0.3)e-9 erg/cm2/s during the flare.
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed at 2026-10-25T13:13:55. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 2.5488 deg, DEC = 60.7658 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data is received. Multi-wavelength observations are encouraged.
Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.